selftests/resctrl: Close perf value read fd on errors

Perf event fd (fd_lm) is not closed when run_fill_buf() returns error.

Close fd_lm only in cat_val() to make it easier to track it is always
closed.

Fixes: 790bf585

 ("selftests/resctrl: Add Cache Allocation Technology (CAT) selftest")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Tested-by: Shaopeng Tan (Fujitsu) <tan.shaopeng@fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>